Revista de Gestão Costeira Integrada
Volume 14, Número 1, Março 2014, Páginas 129-148

DOI: 10.5894/rgci452
* Submission: 6 November 2013; Evaluation: 9 December 2014; Reception of revised manuscript: 4 March 2014; Accepted: 8 March 2014; Available on-line: 18 março 2014

Geochemical characteristics of sediments along the margins of an atlantic-mediterranean estuary (the Guadiana, Southeast Portugal): spatial and seasonal variations *

Características geoquímicas das margens dum estuário Atlanto-Mediterrânico (o Guadiana, Sudeste de Portugal): variação espacial e sazonal

Sarita Camacho ^{@, 1}, Delminda Moura ¹, Simon Connor ^{1, 2},
Tomasz Boski ¹, Ana Gomes ¹

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@ - Corresponding author: scamacho@ualg.pt
1 - CIMA, Faculdade de Ciências e Tecnologia, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
2 - School of Geography and Environmental Science, Faculty of Arts, Monash University, Clayton 3800, Australia

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ABSTRACT
The present work describes spatial and seasonal variability in grain-size, pH and elemental characteristics (TOC, TIC, TN and C/N) in superficial sediments, as well as salinity, temperature and oxygen in the water, within the intertidal range of the Guadiana Estuary, SW Iberian Peninsula, during the year 2010. The results indicate that environmental parameters in the estuary are strongly dependent on spatial patterns, which reflect seasonal oscillations in freshwater discharge. The sediment is generally poorly sorted, with a symmetrical to very finely skewed distribution, in accordance with the low-energy conditions typical of the deposition areas. During winter, heavy rains forced the continuous discharge of Europe’s largest reservoir, the Alqueva dam, creating exceptional hydrodynamic conditions and causing coarser sediment deposition in the estuary. High marsh areas are controlled by flood tides, promoting vertical accretion of muddy sediments, especially silts. The lower areas of the saltmarsh and-mud flat areas are controlled by the ebb, with more efficient export of fines toward the platform and retention of sands, sometimes with significant amounts of bioclasts. In winter (average Q = 654 m3 s-1), salinity remained low throughout the estuary with the highest values (< 3) recorded up to 3 km inland from the river mouth, except in the most confined environments subject to greater evaporation. In summer (average Q = 52 m3 s-1), the estuarine waters are better mixed and significant saltwater intrusion extends approximately 8 km inland. The seasonal variation in surface water temperature was very high, with a difference of ca 14 º C in the maximum temperature recorded in winter and summer.
A clear relationship between elevation (in relation to mean sea level) and organic matter, pH and particle size was observed. In the higher areas of the marsh, where the hydrodynamics is attenuated, differential deposition of fine sediments promotes organic matter entrapment and low pH. Sedimentary organic matter derives from a mixture of native aquatic and terrestrial sources. The mid-upper estuary areas and higher zones of the saltmarsh in the lower estuary incorporate a greater terrestrial component, whereas the low-middle marsh areas of the lower estuary experience a substantial contribution from indigenous aquatic sources.
The present data help to understand the present environmental condition of the Guadiana Estuary and contribute baseline data for future climatic and environmental management studies based on sediment-dependent proxies.

Keywords: Guadiana Estuary; saltmarsh; sediments; granulometry; organic matter; pH; physico-chemical proprieties.

RESUMO
O presente estudo descreve a variabilidade espacial e sazonal das propriedades físico-químicas da água (salinidade, temperatura e oxigénio) e dos sedimentos superficiais (granulometria, pH, COT, CIT, NT e C/N) da faixa intermareal do estuário do rio Guadiana, SW da Península Ibérica durante o ano 2010. Os resultados indicam que as condições ambientais do estuário estão fortemente dependentes dos padrões espaciais e das oscilações sazonais na descarga de água doce. Os sedimentos apresentaram-se geralmente mal calibrados e com uma assimetria tendencialmente fina, de acordo com as condições de baixa energia típicas de áreas de deposição. As fortes chuvas sentidas durante o inverno forçaram a descarga contínua do maior reservatório artificial da Europa, o Alqueva, criando condições hidrodinâmicas excecionais, promovendo o aumento de sedimentos grosseiros no estuário. A granulometria dos sedimentos ao longo da zona intertidal tende a aumentar das zonas mais elevadas às zonas mais baixas. As zonas de sapal alto são controladas pela enchente, promovendo a acreção vertical de sedimentos lodosos, sobretudo siltes. As zonas mais baixas do sapal e zonas de planície lodosa são controladas pela vazante, com exportação mais eficiente de finos para a plataforma e retenção de areias, por vezes com conteúdo significativo em bioclastos. No inverno (média Q = 654 m3 s-1), a salinidade manteve-se baixa ao longo de todo o estuário, com os valores mais elevados (< 3) registados até cerca de 3 km para montante a partir da foz do rio, exceto nos ambientes mais confinados, mais expostos à evaporação. No verão (média Q = 52 m3 s-1), as águas estuarinas apresentaram-se melhor misturadas e a intrusão salina estendeu-se até cerca de 8 km para montante. A variação sazonal da temperatura das águas superficiais foi muito elevada, verificando-se uma diferença de ca 14ºC entre as temperaturas máximas registadas no Inverno e no Verão.
No estuário, observou-se uma clara relação entre a elevação (relativa ao nível médio do mar) e a quantidade de matéria orgânica, o tamanho do grão e o pH. Nas zonas mais elevadas do sapal, onde o hidrodinamismo é atenuado, a deposição diferencial de sedimentos finos promove o aprisionamento de matéria orgânica e a diminuição do pH.
A matéria orgânica sedimentar deriva de uma mistura de matéria orgânica aquática e terrestre de origem autóctone. As zonas do estuário médio-superior e as zonas mais elevadas do sapal do estuário inferior incorporam uma maior componente orgânica de origem terrestre, enquanto que as áreas de baixo-médio sapal do estuário inferior experienciam uma contribuição substancial proveniente de fontes aquáticas de origem alóctone.
A presente informação contribui para o melhor entendimento da atual condição ambiental do Estuário do Rio Guadiana e fornece dados de base para futuros estudos de gestão ambiental e evolução climática que assentem em indicadores ambientais sedimentares.

Palavras-chave:Estuário do Guadiana; sapal; sedimentos; granulometria; matéria orgânica; pH; propriedades físico-químicas.

 

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